Recent industrial tests of amorphous alloys under realistic working environments have indicated that the wear and corrosive resistances of this new category of alloys are at least one order of magnitude higher than that of conventional alloys currently in use. Other amorphous metal compounds are of interest as superconductors at low temperature and as magnetically soft alloys, etc.
Metallic glasses or, equivalently, amorphous metallic alloys can be formed by rapid cooling of liquid metals, or deposition of metallic vapors at rates sufficient to bypass crystallization. For the formation of a metallic glass, cooling rates in the range 10.sup.4 -10.sup.12 K/s are required to suppress nucleation and growth of more stable crystalline phases in undercooled alloy melts. These facts lead to severe restrictions in the synthesis of glassy metals. For example, simple heat transfer considerations require at least one of the specimen dimensions to be rather small, typically 10.mu.-100.mu..
The earliest glassy alloys were manufactured by the splat cooling, gun technique, in which a small quantity of molten alloy was expelled by a shock wave onto a stationary or moving quenching substrate. The shock wave rapidly fragments the melt into tiny droplets which cool to form flake-like products. All subsequent methods have analogous counterparts to splat cooling in that they involve quenching of a high-temperature phase such as a liquid or a vapor phase. Up to the present invention, glassy metal alloys have been made by rapid solidification. Rapid solidification has been achieved by imposing a high undercooling to a melt prior to solidification or by imposing a high velocity of advance to the melt-solid interface during continuous solidification. The undercooling method is limited by the fact that the large supercooling required can only be achieved in the absence of nucleating agents which is difficult to achieve with large melts and is especially hard to achieve for the more reactive metals and alloys. The high-velocity-of-advance technique is limited by heat flow constraints which set in at a cross-section dimension of a few mm.
The production methods all require a primary stage of generating and quenching the melt and, if necessary, a secondary stage of consolidating the product into a useful form. The primary stage requires rapidly bringing a melt of small cross-section into good contact with an effective heat sink. Several methods have been developed which can be classified as spray methods, chill methods and weld methods.
The spray techniques are preferable to the other methods since the cooling rate is rapid before, during and after solidification, increasing the likelihood of retaining the glassy microstructure of the quenched, amorphous material. However, the spray methods are inefficient from an energy standpoint, provide very small sized product which must be further processed by consolidation or dispersed in a matrix resin to form a useful composite.